As electronic devices, such as car air-conditioners, have recently had high performance, variable resistors is demanded to be capable of providing precise resistance output used for operating of the devices.
FIG. 8 is a sectional view of conventional rotary variable resistor 5001. FIG. 9 is an exploded perspective view of variable resistor 5001. Resistor board 1 includes an insulating substrate made of laminated substrate made of paper phenol resin or glass epoxy resin having circular hole 1A formed in the center thereof. Resistor element 92 having a C-shape is provided on a top surface of resistor board 1 at an outer periphery of board 1. Conductor 3 having a ring shape is provided on the top surface and inside the C-shape of resistor element 92. Resistor element 92 and conductor 3 are formed by, for example, screen printing. Both ends of resistor element 2 and conductor 3 are electrically connected to terminals 4A, 4B, and 4C, respectively. Terminals 4A, 4B and 4C are fixed to resistor board 1. Insulating case 5 made of insulating resin has an upper opening having a circular shape. Case 5 has hole 5A having a size and a position substantially identical to those of engaging hole 1A is formed in the center of the circular shape of the upper opening. Resistor board 1 is formed by insert molding so that resistor element 2 and conductor 3 expose on an inner bottom surface of the opening in insulating case 5. Terminals 4A, 4B, and 4C project outward from a side wall of plate portion 5B extending from the side wall of insulating case 5. Rotating body 6 has through-hole 6A having an oblong shape provided in the central portion thereof Rotating body 6 has flange 6B having a dusk shape in the upper portion of body 6, and shaft 6C having a cylindrical shape in the lower portion of body 6. Slider 7 is fixed to a bottom surface of flange 6B. Slider 7 slides on and contact resistor element 2 and conductor 3 provided on resistor board 1. Shaft 6C having the cylindrical shape is inserted in hole 1A of resistor board 1 and engaging hole 5A of insulating case 5 which overlap each other. Shaft 6C has a thin tip. The tip is formed to have a trumpet shape on the bottom surface of insulating case 5 and attached rotatably to the bottom surface of case 5.
An operation of rotary variable resistor 5001 will be described below. A rotation operator is inserted in through-hole 6A through rotating body 6 having a substantially oblong shape. When rotating body 6 rotates, slider 7 fixed to the bottom surface of flange 6B slides on resistor element 92 and conductor 3 provided on resistor board 1. A resistance in response to a position at which slider 7 contacts resistor element 2 is obtained from terminals 4A, 4B, and 4C.
Conventional rotary variable resistor 5001 may output a resistance varying from a predetermined resistance due to variations of resistor element 2 printed on resistor board 1 or displacement of components.
Conventional variable resistor 5001 may reduce production yield rate of devices, such as vehicle-mounted devices, and audio visual devices, which require a precise resistance, and raises their production cost.